Go to Chase's website and click on Products & Services, then on Credit Cards and Customer Service. Here you will find several links and options to report your card lost.
As of 2014, Chase provides a general customer service toll-free number on this page, or you may scroll down to the "Frequently Asked Questions (FAQs)" heading on the lower left of the page. Under this, click on the link, "How do I report a lost or stolen credit card?" This brings up a pop-up window with a direct toll-free number to report your card stolen.
After you have called Chase to report your card lost or stolen, change your personal identification number, or PIN.